Cassegrain antenna



J1me 1966 G. VON TRENTlNl 3,

CASSEGRAIN ANTENNA Filed July 24, 1962 United States Patent 3,255,455 CASSEGRAIN ANTENNA' Giswalt Von Trentini, Mnnich-Solln, Germany, assignor to Siemens & Halske Aktiengesellschaft, Berlin and Mnnich, Germany, a corporation of Germany Filed July 24, 1962, Ser. No. 212,052 Claims priority, application Germany, July 31, 1961, S 75,014 5 Claims. (Cl. 343-762) The invention disclosed herein is concerned with an antenna for very short electromagnetic waves, comprising a parabola mirror ahead of which is arranged a hyperboloidlike auxiliary reflector having a considerably smaller aperture than the mirror, such reflector cooperating with a primary radiator disposed at the apex of the parabola mirror and serving for flooding of the mirror.

An antenna of this type is also known as Cassegrain Antenna. The primary radiator is in such case often constructed as a funnel-like radiator with relatively very small aperture, with the radiation thereof directed to the noticeably larger hyperboloid-like auxiliary reflector from which the radiation passes to the parabola mirror proper. The radiation path is upon receiving oppositely directed. The operation of such antenna is considerably more favorable than that of a parabola mirror antenna which is fed directly by a horn radiator. The focal point of the large parabola mirror coincides with the focal point lying in back of the auxiliary reflector and the second focal point of the hyperboloid-like auxiliary reflector falls in the phase center of the primary radiator. Such antennas have, with strong focusing and great focal distance, the disadvantage that the primary radiator focuses likewise relatively strongly, therefore requiring a large aperture. The primary radiator, constructed as .funnel radiator, has for this reason great length. It is moreover in most cases diflicult to obtain good matching, good efliciency and a good radiation diagram in an extended frequency range.

The object of the invention is to improve a Cassegrain antenna particularly with respect to the above noted drawbacks.

Proceeding from an antenna for very short electromagnetic waves, comprising a parabolic mirror having with respect to the waves a very large aperture, ahead of which is arranged a hyperboloid-like auxiliary reflector with con siderably smaller aperture, said auxiliary reflector cooperating with a primary radiator disposed at the apex of the parabola mirror to reflect waves from said primary radiator to said mirror, the object of the invention being realized by constructing the primary radiator as a horn I parabola antenna having, as compared with the aperture of the parabola mirror, a small aperture, such primary radiator radiating from the rear of the parabola mirror through the apex thereof and being directed to the hyperboloid-like auxiliary reflector the spacing of which, from the parabola mirror apex, corresponds approximately to one-half up to the entire Rayleigh spacing, and the aperture of which is equal to or somewhat larger than that of the horn parabola antenna.

It is desirable, in the case of a movable parabola mirror antenna for the reception of radiation from space or for effecting radiation into space, that the parabola mirror be arranged for rotation upon tracks, horizontally to the surface of the earth and for swinging or pivotal motion about an axis of rotation extending in back of the parabola mirror perpendicularly to the axis thereof.

It is moreover of advantage to arrange the cone of the horn parabola antenna radially in the direction of the pivot axis and to connect it with the given high frequency devices over a rotary coupling disposed preferably in the vicinity of the journal for the pivotal motion. This also applies in the event that the parabola mirror does not ex- 3,255,455 Patented June 7, 1966 tend parallel to the surface of the earth. It is also desirable that the hyperboloid-like auxiliary reflector be supported with respect to the parabola mirror, preferably by means of three strut-like supports.

An antenna of this type is of particular advantage in case of an arrangement in which a sender and/ or a preferably extremely noiseless receiver are to be connected with the terminal means of the horn parabola antenna, as it meets requirements posed with respect to short leads, small line losses and good matching in a broad frequency band.

An embodiment of the antenna according to the invention will now be described with reference to the accompanying drawing.

The illustrated antenna comprises a parabola mirror 1 with relatively very large diameter, measuring, for example, from twenty to thirty meters, which is positioned upon the surface of the earth by means of a mounting provided with rotary tracks 2, 3. The parabola mirror 1 is thus rotatable about the axis 4 and may also be swung or pivoted about the .axis 6 by means of a pivot journal 5.

The horn parabola antenna, serving as primary radiator for the parabola mirror, comprises a parabola radiator 7 which merges into a hollow cone 8 of known construction. The aperture 9 of the horn parabola antenna is trained upon the hyperboloid-like auxiliary reflector 10 having an aperture which is preferably somewhat larger than the aperture 9 of the horn parabola antenna. The connection of the root of the cone 8 with the high frequency devices 11, including a sender S and a receiver E, which are fixedly disposed, preferably rigidly fastened upon the rotation track and thus moving along with the parabola mirror 1 only upon rotation thereof about the axis 4, is effected by means of the rotary coupling 12 which is arranged in the vicinity of the pivot journal 5.

The following is important insofar as the electrical di rnensioning of the arrangement is concerned. The spacing between the aperture 9 of the horn parabola antenna and the auxiliary reflector 10 corresponds approximately to one-half up to the full Rayleigh spacing. The term Rayleigh spacing signifies the distance resulting from the equation R=D /2 wherein D is the diameter of the aperture 9 and A the ope-rating wave length. The curvature of the hyperboloid-like auxiliary reflector 10 is dimensioned so that there results for the hyperboloid, as a second focal point, in back of the parabola mirror antenna 1, a finite focal point which is so selected that the phase front of the parabola mirror antenna 1 is at least nearly plane. 1

For example, in thecase of an operating range between 4 and 6 gigacycles, and assuming a parabola mirror 1 with a diameter of 20 meters and an aperture 9 with a diameter of 1 meter, there may be provided an auxiliary reflector with an aperture diameter amounting to 1.2 meters, with the auxiliary reflector 10 spaced by about 5.5 meters from the mirror apex, ata focal distance of the auxiliary reflector 10 on the order of about 20 to 30 meters.

The horn parabola, the parabola part of which may also be a section of an ellipsoid, is, insofar as the feed cone is concerned, advantageously made as follows: The rotary coupling 12 is disposed in the vicinity of the pivot journal 5, the cone 8 extending continuously and as linearly the hyperboloid-like auxiliary reflector. The field strength of the waves reflected at the hyperbol-oid shall strongly decrease toward the mirror margin in order to keep the side maxima of the antenna as small as possible. The side maxima lying farther away from the main maximum of the parabola mirror proper are in the described embodiment already small owing to the relatively low-ratio of the mirror focal length of the parabola mirror 1 to the diameter thereof, such ratio being in the assumed case about 0.25. This also applies for the radiation in reverse direction. A further decrease, if desired, may be obtained by providing a cylindrical extension disposed ahead of the parabola mirror or by providing an asymmetrically formed diaphragm therefor. It is also feasible to affect in this sense the current loading, by the use of blocking circuits or grids. A type of a known apex plate may be arranged ahead of the hyperboloid-like auxiliary reflector 1a in order to improve the matching.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. An antenna for very short electromagnetic waves, comprising a parabola mirror with an aperture which is very large as compared with the waves, a hyperboloid-like auxiliary reflector having an aperture which is considerably smaller than that of the mirror, a primary radiator disposed at the back of said mirror adjacent the apex of the latter, said auxiliary reflector being cooperable with said primary radiator for reflecting said waves from the latter to said mirror, said primary radiator comprising a horn parabola having an aperture which is small as compared with the aperture of the parabola mirror, the funnel portion of said'horn parabola extending radially with respect to the mirror axis, and the radiation of said horn parabola extending from the back of the parabola mirror through the apex thereof and being trained upon said by perboloid-like auxiliary reflector, the spacing of the auxiliary reflector from the apex of the parabola mirror corresponding to about one-half up to the full Rayleigh spacing, and the aperture of said auxiliary reflector being approximately equal to or somewhat larger than the aperture of said horn parabola.

2. An antenna according to claim 1, wherein the curvature of the hyperboloid-like auxiliary reflector is such that there results for the hyperboloid, as a second focal point, in back of the parabola mirror, a finite focal point which is so disposed that the phase front of the parabola mirror is at least nearly plane. 1

3. An antenna according to claim 1, wherein the funnel of said horn parabola is a cross-sectionally circular cone.

4. An antenna according to claim 1, wherein said parabola mirror is arranged for rotation upon a track positioned in a plane extending horizontally with respect to the surface of the earth, journal means disposed in back of said parabola mirror and forming a pivot axis which extends in parallel With the plane of said track, and means for mounting said parabola mirror on said journal for pivotal motion thereof perpendicularly to its own axis.

-5. An antenna according to' claim 4, wherein said horn parabola is provided with a funnel extending radially in the direction of said pivot axis, and coupling means disposed in the vicinity of said journal means for rotatably coupling the end of said funnel with high frequency de vices provided for cooperation with said antenna.

References Cited by the Examiner UNITED STATES PATENTS 2,032,58 8 3/1936 Miller 343-837 2,817,837 12/1957 Dale 343775 2,996,716 8/1961 Witherby 343761 3,072,905 1/1963 Wilkes 343761 FOREIGN PATENTS 861,718' 1/1953 Germany.

OTHER REFERENCES Sciambi, Jr; RCA Technical Notes, March 1962, RCA TN. No. 549; pages 1 and 2.

HERMAN KARL SAALBACH, Primary Examiner.

W. K. TAYLOR, E. LIEBERMAN, Assistant Examiners. 

1. AN ANTENNA FOR VERY SHORT ELECTROMAGNETIC WAVES, COMPRISING A PARABOLA MIRROR WITH AN APERTURE WHICH IS VERY LARGE AS COMPARED WITH THE WAVES, A HYPERBOLOID-LIKE AUXILIARY REFLECTOR HAVING AN APERTURE WHICH IS CONSIDERABLY SMALLER THAN THAT OF THE MIRROR, A PRIMARY RADIATOR DISPOSED AT THE BACK OF SAID MIRROR ADJACENT THE APEX OF THE LATTER, SAID AUXILIARY REFLECTOR BEING COOPERABLE WITH SAID PRIMARY RADIATOR FOR REFLECTING SAID WAVES FROM THE LATTER TO SAID MIRROR, SAID PRIMARY RADIATOR COMPRISING A HORN PARABOLA HAVING AN APERTURE WHICH IS SMALL AS COMPARED WITH THE APERTURE OF THE PARABOLA MIRROR, THE FUNNEL PORTION OF SAID HORN PARABOLA EXTENDING RADIALLY WITH RE- 